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The shelf‐margin deltas are the primary 'sources' of interest in siliceous submarine fan source‐to‐sink systems. However, less has yet to be discovered about the roles and effects of the shallow water platform situated on continental slopes. Using the Miocene sediments of the Ledong‐Lingshui Sag in the western part of the Qiongdongnan Basin (QDNB), this paper studies the YC35 coarse‐grained submarine fan that developed in the Meishan Formation (Fm.). The material sources, depositional characteristics, and developmental mechanisms of this fan were investigated. The restored palaeogeomorphology, based on high‐resolution 3D seismic data, shows that a shallow water platform exists. According to geochemical palaeoenvironmental data, the shallow water platform was ideal for forming carbonate deposits in the Meishan Fm., owing to the warm and humid palaeoclimate and the shifting trend in palaeowater depth. Based on the combined source tracing of heavy minerals, coherence slices, and seismic profiles, the shallow water platform and the Ningyuan River source on Hainan Island contributed to the YC35 coarse‐grained submarine fan. Compared to submarine fans formed during different periods, coarse‐grained fan stands out due to its distinct sedimentary structure, rock composition, and microlithological characteristics. Aside from not following the usual Bowmar sequence, there is an excess of gravel and no clear bedding or lamination. A large number of rock fragments, mostly granite and sedimentary rock (carbonatite), make up the rock composition. According to these results, the shallow water platform significantly affects submarine fan material composition. In addition, the attribute slice based on the root mean square reveals that shallow water platforms moderate peripheral deposition. We attribute the development and proximal supply of the shallow water platform to diapirism and forced regression. Our research provides novel insights and comprehension into the investigation of submarine fan sedimentary systems. [ABSTRACT FROM AUTHOR]

As the interface between seawater and the seabed, superficial sediments on the seabed are an important part of the marine acoustic field environment and are indispensable for marine resource investigations. Studying sediments several meters to hundreds of meters below the seafloor is highly valuable and important. This study processes and analyses the water depth, topography and bottom data and obtains the shallow bottom profile and topographic map of the northern continental slope of the South China Sea (SCS). The study analyzes the influence of physical parameter (including density, porosity, and grain size) on the acoustic velocity in sediments. Single-parameter and dual-parameter models are established to further examine this influence. The results show that porosity and density have greater influences on the acoustic velocity of sediments than does grain size. Finally, the acoustic properties of several typical stations with water depths are tested to analyze the variations in the acoustic properties of the shallow sediments in the northern SCS. The results show that the influence of each parameter on the prediction of the acoustic velocity of the sediment is in the following order: porosity>density>grain size. This study analyses and reveals the reason why the seafloor sediments in the local area cause the acoustic properties to change greatly. It may be caused by changes in the sediment type, lithology along with the depth. And the other reason is the development of interlayer in the land slope of the northern SCS. [ABSTRACT FROM AUTHOR]

The acoustic properties of seafloor sediments on continental slopes play a crucial role in underwater acoustic propagation, communication, and detection. To investigate the acoustic characteristics and spatial distribution patterns of sediments on the continental slope, a geoacoustic experiment was conducted in the northwestern South China Sea. The experiment covered two sections: one crossing the shelf and slope in the downslope direction, and the other near the shelf break in the along-slope direction. In situ techniques, sediment sampling, and laboratory measurements were used to acquire data on sediment acoustic properties (such as sound speed and attenuation) and physical properties (including particle composition, density, porosity, and mean grain size). The experimental findings revealed several key points: (1) Acoustic properties of shallow water coarse-grained sediments and deep-sea sediments were higher when measured in the laboratory compared to in situ measurements. (2) Relationships between measured attenuation and physical properties, as well as between sound speed and mean grain size, showed deviations from previous empirical equations. (3) Sediment acoustic and physical properties exhibited significant variations in the downslope direction, while showing gradual variations in the along-slope direction. These variations can be attributed to sedimentary environmental factors such as material sources, hydrodynamic conditions, and water depth. [ABSTRACT FROM AUTHOR]

Using observations and theoretical models, a substantial topographic modulation on the quasigeostrophic (QG) dynamics, which results in a primary surface mode distinct from the classic first baroclinic (BC1) mode with a flat bottom, is revealed in the northern South China Sea (NSCS). In contrast to open oceans, the surface-intensified modes decay downward more rapidly over the continental slope of the NSCS, with a mean e-folding scale of approximately 1/5 of water depth. The subinertial flow variability appears to be vertically incoherent, with planetary and topographic Rossby waves dominating in the upper and deep layers, respectively. With a larger deformation radius (Rd), the surface-mode Rossby waves propagate at a phase speed ~1.5 times of that of the BC1 mode. Moreover, the modal structures can be substantially modified by seasonal NSCS circulation, which is significantly enhanced over continental slopes. Analysis of the triad interactions further implies that the short waves tend to transfer energy to larger scales via the inverse cascade and only those with wavelengths larger than Rd ≈ 70 km in the NSCS can persist because of a slower unstable growth rate but a higher fraction of upscale energy transfer. The present theory excludes the bottom-trapped mode, which is closely associated with topographic Rossby waves and is observed to be significant in the abyssal NSCS. Hence, a complete normalmode basis for any QG state is required for a study that focuses on flow variability throughout the water column. Our findings provide an insight into the vertical partition of horizontal kinetic energy for QG motions, as well as the relevant oceanic variation in marginal seas. [ABSTRACT FROM AUTHOR]

The southwestern tropical Pacific exhibits a complex bathymetry and represents a hot spot of internal-tide generation. Based on a tailored high-resolution regional model, we investigate for the first time the internal-tide field around the New Caledonia islands through energy budgets that quantify the coherent internal-tide generation, propagation, and dissipation. A total of 15.27 GW is converted from the barotropic to the baroclinic M2 tide with the main conversion sites associated with the most prominent bathymetric structures such as continental slopes and narrow passages in the north (2.17 GW) and ridges and seamounts south of New Caledonia (3.92 GW). The bulk of baroclinic energy is generated in shallow waters around 500 m depth and on critical to supercritical slopes, highlighting the limitations of linear semi-analytical models in those areas. Despite the strongly dominant mode-1 generation, more than 50 % of the locally generated energy either dissipates in the near field close to the generation sites or loses coherence. The remaining baroclinic energy propagates within well-defined tidal beams with baroclinic energy fluxes of up to 30 kW m -1 toward the open ocean. The New Caledonia site represents a challenge for SWOT (Surface Water and Ocean Topography) observability of balanced motion in the presence of internal tides with sea surface height (SSH) signatures >6 cm at similar wavelengths. We show for our study region that a correction of SSH for the coherent internal tide potentially increases the observability of balanced motion from wavelengths >160 km to well below 100 km. [ABSTRACT FROM AUTHOR]

The TEX86 paleothermometer has been extensively used to reconstruct past sea water temperatures, but it remains unclear which export depths the proxy represents. Here we used a novel approach to better constrain the proxy recording depths by investigating paleotemperature proxies (TEX86, U37K′ ${\mathrm{U}}_{37}^{{\mathrm{K}}^{\prime }}$, RI−OH and RI−OH′) from two pairs of proximal (<12 km apart) cores from Chilean and Angola margins, respectively. These cores are from steep continental slopes and lower shelves, which leads to a substantial difference in water depth between them despite being closely located. Surprisingly, the deep and the shallow U37K′ ${\mathrm{U}}_{37}^{{\mathrm{K}}^{\prime }}$ records at the Chilean margin show dissimilarities, in contrast to the similar records from the Angola margin, which may be due to post‐depositional alteration at the former sites. In contrast, the TEX86 records were statistically indistinguishable between the sites at both the locations, even though the GDGT [2]/[3] ratio suggests GDGTs derived from potentially different archaeal communities residing at different depths. A short‐lived difference between the TEX86 records is observed during the last glacial period at the Angola margin, possibly due to a contribution of Antarctic Intermediate Waters to the deep site. Modelling suggests that the TEX86 source signal at our core sites reaches its peak abundance at water depths shallower than 350 m. The RI−OH and RI−OH′ records show similar variability as the TEX86 records, although regional differences in their absolute temperature estimates exist. Our approach using proximal sediment cores at steep slopes appears useful to constrain the export depth of organic proxy signals for paleo‐reconstructions. Key Points: Source signal of TEX86 proxy is likely derived from depths shallower than 350 m at Chilean and Angola marginsOH‐GDGT based proxies show similar variability as TEX86 records, but show regional differences in their absolute temperature estimatesU37K′ ${\mathrm{U}}_{37}^{{\mathrm{K}}^{\prime }}$ proxy signal may be impacted by post‐depositional changes or lateral transport even for closely located cores [ABSTRACT FROM AUTHOR]

Seafloor methane emissions can affect Earth's climate and ocean chemistry. Vast quantities of methane formed by microbial decomposition of organic matter are locked within gas hydrate and free gas on continental slopes, particularly in large areas with high sediment accumulations such as deep-sea fans. The release of methane in slope environments has frequently been associated with dissociation of gas hydrates near the edge of the gas hydrate stability zone on the upper slope, with discharges in greater water depths less understood. Here we show, using data from the Rio Grande Cone (western South Atlantic), that the intrinsic, gravity-induced downslope collapse of thick slope sediment accumulations creates structures that serve as pathways for gas migration, unlocking methane and causing seafloor emissions via giant gas flares in the water column. The observed emissions in the study region (up to 310 Mg year−1) are three times greater than estimates for the entire US North Atlantic margin and reveal the importance of collapsing sediment accumulations for ocean carbon cycling. Similar outgassing systems on the Amazon and Niger fans suggest that gravity tectonics on passive margins is a common yet overlooked mechanism driving massive seafloor methane emissions in sediment-laden continental slopes. [ABSTRACT FROM AUTHOR]

The Southwestern Tropical Pacific exhibits a complex bathymetry and represents a hot spot of internal tide generation. Based on a tailored high-resolution regional model, we investigate for the first time the internal tide field around the New Caledonia islands through energy budgets that quantify the internal tide generation, propagation, and dissipation. A total of 15.97 GW is converted from the barotropic to the baroclinic tide with the main conversion sites associated with the most prominent bathymetric structures such as continental slopes and narrow passages in the north (2.17 GW) and ridges and seamounts south of New Caledonia (3.92 GW). The bulk of baroclinic energy is generated in shallow waters around 500 m depth and on critical to supercritical slopes highlighting the limitations of linear semi-analytical models in those areas. Despite the strongly dominant mode-1 generation, more than 50 % of the locally generated energy dissipates in the near-field close to the generation sites. The remaining energy propagates within well-defined tidal beams with baroclinic energy fluxes of up to 30 kW m−1 toward the open ocean, strongly dominated by mode-1. The energetic mesoscale eddy activity in the region appears to be the main source of tidal incoherence. Locally, mesoscale eddy-driven stratification changes induce variations of the conversion term. In the far-field, incoherence of the energy flux arises through the interaction of the tidal beam with the eddying background flow. The New Caledonia site represents a challenge for SWOT (Surface Water Ocean Topography) observability of meso- and submesoscale dynamics in the presence of internal tides with sea surface height signatures > 6 cm. We show that a correction of the coherent baroclinic tide may improve the observability range by shifting the transition scale between balanced and unbalanced flow in winter from 180 km to 50–80 km. In contrast, in summer observability increases only marginally due to the seasonally amplified signature of the incoherent tide at scales below 100 km. [ABSTRACT FROM AUTHOR]

Enclosed depressions, termed pockmarks, are widespread seafloor morphologies, commonly associated with fluid seepage. This study provides the first detailed documentation of pockmarks offshore the South Westland margin of the South Island/Te Waipounamu, Aotearoa New Zealand. Pockmarks are identified from multibeam bathymetry (25-m grid) through manual and semi-automated selection in water depths of 100–2600 m. Pockmarks are most concentrated at 400–850 m water depth on continental slope areas between submarine canyons. A continuum of pockmark morphologies includes – (1) large (>0.5 km2 area) and irregularly shaped pockmarks above partially infilled channels; (2) small and circular pockmarks (∼100–200 m diameter; ∼0.008–0.03 km2 area) occurring between canyons; and (3) elongated and intermediate size pockmarks, generally oriented along-slope and often occurring above buried sediment waves. Elongated pockmarks appear to have been modified by near-seafloor oceanographic and/or turbidity current flows. Pockmark features occur across many locations around Aotearoa, including both the eastern and western margins. Some similar pockmark morphologies are identified in these different tectonic, sedimentary, and oceanographic settings, suggesting that there may be some similarity in formative mechanisms, but clear mechanisms leading to their formation remain enigmatic. [ABSTRACT FROM AUTHOR]

This study aimed to characterize the spatial distribution and composition of living Benthic Foraminifera (BF) and to comprehend how environmental conditions (e.g., organic matter) can affect communities of these protozoa in the northern and southern sectors of the Santos Basin (SB), in the continental slope and São Paulo Plateau. In this context, 23 stations (65 samples including replicates at each station) were collected between 400 and 2,400 m water depth. Multivariate analyses revealed that the ecological structure of the community changes mainly along the bathymetric gradients. Stations located between 400 and 700 m, both in northern and southern sectors, are characterized by the presence of indicator species of high intensity of currents, such as Globocassidulina subglobosa and Trifarina bradyi. These stations are also mainly marked by the occurrence of Epistominella exigua, a phytodetritivore species. The stations at 1,000 and 1,300 m depth, in both sectors, are characterized by high accumulation of organic matter in the sediments, which favors the development of agglutinated foraminifera species, such as those of the genus Reophax. Finally, the lower slope and the São Paulo Plateau, in both sectors, are oligotrophic regions, with pulses of labile organic carbon, probably low current velocities and the presence of Alabaminella weddellensis. The quantity and quality of food, which are closely related to hydro-sedimentary dynamics and bentho-pelagic coupling in the slope and São Paulo Plateau, are the main factors that influence the distribution of living BF assemblages in the SB. [ABSTRACT FROM AUTHOR]

Mooring observations of nearly the full water column on the continental slope in the South China Sea revealed the ocean's response to Typhoon Mangkhut (2018). Mangkhut induced sea surface cooling ∼4°C that was biased to the right side of its track, which recovered with an e‐folding time after approximately 1 week. Mangkhut was a relatively fast‐moving typhoon and caused a fast near‐inertial response throughout the entire depth in its lee. The typhoon‐induced upper ocean (deep‐water) near‐inertial current velocities were >1.5 m/s (∼0.08 m/s), with an e‐folding time of approximately a week (2 weeks) and frequency of 1.04f (1.08f, where f is the local inertial frequency). The near‐inertial currents were near‐circular polarized in the upper ocean and near‐rectilinear polarized with the main axis in the across‐slope direction in deep water. The deep‐water near‐inertial waves amplified the vertical excursions of isotherms from ∼120 to ∼200 m, reduced the stratification, elevated vertical current shears, and enhanced turbulent dissipation rate, especially during 14–17 September when the effects of near‐inertial waves and diurnal spring tides overlapped. A net cooling ∼0.15°C and salinity increase ∼0.05 psu were observed in the deep ocean after Mangkhut. Typhoon‐induced near‐inertial waves may further contribute to the intensification of southwestward along‐slope mean near‐bottom currents. This study indicates the immediate influence of typhoon in deep‐water through near‐inertial waves and their contribution to the bottom mixing on the continental slope. Plain Language Summary: In 2018, Typhoon Mangkhut traveled over a moored observation station on the continental slope in the South China Sea. Mangkhut caused rightward biased sea surface cooling ∼4°C and recovered nearly a week. Mangkhut was a relatively fast‐moving typhoon, generating near‐inertial waves throughout the entire water depth, with the maximum current speed >1.5 m/s (∼0.08 m/s) in upper ocean (deep water). These waves recovered in almost a week (2 weeks). The near‐inertial currents were similar in all the horizontal directions in upper ocean, while the across‐slope currents were much greater than the along‐slope currents in deep water. The deep‐water near‐inertial waves increased the vertical variation of temperature and salinity isolines from ∼120 to ∼200 m, made near‐bottom water more homogeneous, elevated vertical current difference and enhanced the turbulence, especially when near‐inertial waves and diurnal tides were both affected. A net cooling of ∼0.15°C and salinity increase of ∼0.05 psu were observed in the deep ocean after Mangkhut. Typhoon‐induced near‐inertial waves may further contribute to the intensification of southwestward along‐slope near‐bottom mean flow. This study indicates that the typhoon immediately influence deep water through generating near‐inertial waves and enhancing the bottom mixing on the continental slope. Key Points: Near‐inertial currents were observed in the whole water column on a continental slope in the South China Sea after Typhoon Mangkhut (2018)The near‐inertial waves were near‐circular polarized in upper ocean and near‐rectilinear polarized in deep oceanLarge vertical isotherm excursions and enhancement of bottom turbulent dissipation rates induced by deep‐water near‐inertial waves [ABSTRACT FROM AUTHOR]

Ischia Island is an active volcano representing the emerged sector of an E -W trending volcanic ridge largely extending undersea. Its collapsing behaviour, mainly in the form of fast-moving, terrestrial and submarine debris avalanches, has been recognized during the Holocene, but much less is known about previous gravity-driven processes. Using high-resolution multibeam bathymetric data and seismic reflection profiles, we provide evidence that the Island's southwestern flank has been involved in a slow-moving, deep-seated slope deformation that has displaced large volumes of its apron since the Late Pleistocene and until very recent or contemporary times. A long tongue of deformed seafloor, spreading up to 45 km from the Island over an area of 330 km2, between 500 and 1300 m water depths, has been detected along its southwestern slope. Different types of mass movements, genetically associated with each other, characterize this landslide: 1) a basal slump anticline, corresponding to a bulge on the bathymetry detaching at about 400 m sub-bottom depth; 2) an intermediate-mass movement chiefly consisting of debris avalanches and debris/turbiditic flows; 3) an upper mass movement consisting of hundred-metre size slumps detaching at relatively shallow depths. Conservative estimates indicate that at least 50 km3 of volcano-clastic and hemipelagic deposits have been mobilized, most of which comprise the basal slump anticline. This submarine landslide can be explained as a gravity failure of the continental slope unrelated to volcanism or rather as a process related to the dynamics of the volcanic edifice, which would imply volcano-spreading. • Oceanic and continental volcanic edifices may undergo slow collapse, named spreading • Ischia is an active volcano suspected to undergo long-term subsidence along with seismicity • Its southern, undersea section is affected by fast and long-term landslide phenomena • This long-term gravity instability can be viewed within the concept of volcano-spreading. [ABSTRACT FROM AUTHOR]

Here we present a record of coccolith variation over the past 14 kyr BP from core KF-02 (25.84°S, 45.2°W; 827 m water depth) collected from the continental slope of the São Paulo Bight (SPB), located in the subtropical western South Atlantic. Additionally, we estimated primary productivity based on Florisphaera profunda nannoliths and compared it with other published investigations on the productivity based on different proxies. The coccolith assemblages varied and agreed with the main regional oceanic and atmospheric changes since the latest deglaciation, likely in response to the terrestrial influence and nutricline depth variations. Among eutrophic species, Gephyrocapsa oceanica was more abundant during times with enhanced productivity promoted by the input of terrestrial material. Emiliania huxleyi and small Gephyrocapsa dominated the middle Holocene samples and probably benefitted from the more stable oceanic conditions and the enhanced light availability promoted by the Brazil Current. The paleoproductivity estimates indicated that moderate levels of productivity persisted since the latest deglaciation with a decreasing trend after the middle Holocene, delaying the productivity reduction that was indicated by other regional studies based on planktonic foraminifera. Apparently, seasonal regional hydrographic conditions were responsible for changing coccolith assemblages, and these were not reflected by the planktonic foraminifera G. bulloides variations. • Paleoproductivity in the subtropical Brazilian margin based on Florisphaera profunda. • Higher levels of primary productivity during late deglaciation in the São Paulo Bight. • Primary productivity declined since the middle Holocene in western South Atlantic. • Late Holocene productivity tends to oligotrophy at the subtropical Brazilian margin. • Seasonal hydrographic variability potentially influenced the coccolith assemblages. [ABSTRACT FROM AUTHOR]

The global supply of Antarctic Bottom Water (AABW) is sourced from a handful of dense overflows. Observations from the Weddell Sea indicate that the overflow there exhibits prominent oscillations accompanied by dense eddies, while the Ross Sea overflow shows no significant oscillations other than tides, yet the genesis of these oscillations and their role in mediating AABW export remain poorly understood. Here idealized model simulations are used to investigate the dynamics of these oscillations. It is shown that the dominant oscillations result from the formation of Topographic Rossby waves (TRWs) associated with baroclinic instability of the dense overflow. A key finding is that the TRWs can feed back onto the dense overflow, producing coherent subsurface eddies of the same frequency. A series of sensitivity experiments reveal that these behaviors depend strongly on the local environment: steep topographic slopes suppress the baroclinic growth of TRWs, while strong downstream along‐slope flows suppress the upstream propagation of TRW energy and genesis of subsurface eddies. These results explain the varying prevalence of different oscillatory phenomena observed across different dense overflow regimes. Plain Language Summary: Antarctic Bottom Water (AABW) comprises the densest water in the major ocean basins, filling more than one‐third of the ocean. AABW is sourced from dense water spilling into the deep ocean at several specific sites around the Antarctic coast, referred to as "overflows." These overflows have been observed to coincide with energetic fluctuations of water across the Antarctic continental slope, which separates the relatively shallow waters around the Antarctic continent from the deep waters of the open ocean. However, the mechanism via which these fluctuations are generated, and their interaction with the overflows of dense waters, remain poorly understood. This study identifies a range of behaviors over the continental slope, ranging from no fluctuations at all, to wave‐like fluctuations across the continental slope, to the formation of horizontal vortices. The transitions between these behaviors are shown to depend on the local environmental conditions, notably the steepness of the continental slope and the speed of the ambient flow along the continental slope. These findings are in good agreement with in‐situ observations and help to explain the range of behaviors observed in overflows in nature. Key Points: Baroclinic instability of dense overflows energizes coupled Topographic Rossby waves (TRWs), which oscillate throughout the water columnFor small topographic slopes, TRWs can feed back to the outflow and instigate the formation of coherent, dense eddiesSteep slopes can suppress the formation of TRWs and eddies, decreasing the amplitude of dense overflow variability [ABSTRACT FROM AUTHOR]

The evolution of halocline waters in the Makarov Basin and along the East Siberian continental slope is examined by combining drifting platform observations, shipborne hydrographic data, and simulations from a global operational physical model from 2007 to 2020. From 2012 onwards, relatively shallow and cold Atlantic‐derived lower halocline waters, previously restricted to the Lomonosov Ridge area, progressed eastward along the East Siberian continental slope. Their eastward extent abruptly shifted from 155°E to 170°E in early 2012, stabilized at 170°E until the end of 2015, then gradually advanced to reach the western Chukchi Sea in 2017. Such eastward progression led to a strengthening of the associated boundary current and to the shedding of mesoscale eddies of cold Atlantic‐derived waters into the lower halocline of the Makarov Basin in September 2015 and near the East Siberian continental slope in November 2017. Additionally, active mixing between upwelled Atlantic Water and shelf water formed dense warm water supplying the Makarov Basin lower halocline. The increasing contribution from Atlantic‐derived waters into the lower halocline along the East Siberian continental slope and in the Makarov Basin led to a weakening of the halocline, which is characteristic of a new Arctic Ocean regime that started in the early 2000s in the Eurasian Basin. Our results suggest that this new Arctic regime may now extend toward the Amerasian Basin. Plain Language Summary: In the Arctic Ocean, the "halocline" is a cold near‐surface layer where salinity increases rapidly with depth. The halocline isolates the sea ice at the surface from the heat stored in the underlying warm and salty water. Hence, the strength of the halocline is a key feature in the maintenance of the sea ice cover. In this study, we use various ocean measurements and computer models to document recent changes in the halocline in the East‐Amerasian sector of the Arctic. In this region, warm and salty water, usually found about a hundred meters deep, was found to become closer to the surface from 2012 onwards. We show that the shoaling of this warm water contributed to the progressive warming and weakening of the halocline in the East‐Amerasian Arctic Ocean. This weakening of the halocline is emblematic of the emergence of a new Arctic regime, previously observed in the Eurasian sector of the Arctic, a region undergoing the growing influence from the Atlantic Ocean. Our study suggests the progression of this new regime, with shallow warm water and weakened halocline, further into the Arctic. Key Points: Ice‐tethered measurements in 2008, 2015, and 2017 in the Makarov Basin highlight changes in the upper and lower halocline propertiesThe cold Atlantic‐derived lower halocline progressed eastward along the East Siberian slope reaching the Chukchi BorderlandThe Makarov Basin lower halocline was increasingly supplied with warmer water resulting from the mixing of Atlantic water with shelf water [ABSTRACT FROM AUTHOR]

In wintertime, a high-density water forms on the shallow shelf in the vast Peter the Great Bay (Japan Sea). The steep continental slope with deep canyons and cold winters in the area provide suitable conditions for the implementation of deep slope convection—an important phenomenon in the formation of intermediate and bottom waters that occurs at a few locations in some semi-enclosed seas, including the Japan Sea. The descent of dense shelf water down the continental slope of Peter the Great Bay usually occurs to 1000–1200 m; however, in anomalously cold winters, it has been observed at greater than 2000 m depth supporting renewal and deep ventilation of intermediate and bottom waters in the Japan Sea. The deep slope convection is a rare episodic phenomenon with durations ranging from several hours to several days, that has never been simulated in Peter the Great Bay with a realistic numerical model of circulation. We apply the Regional Ocean Modeling System (ROMS) with a 600 m horizontal resolution to simulate the deep slope convection in the anomalously cold winter of 2001 when it has been observed in cruises. The results are compared with propagation of deep shelf water in the regular winter of 2010 when hydrological characteristics of this water were recorded by a profiler "Aqualog" installed at the shelf break. Using Lagrangian methods, we track and analyze the formation of dense shelf water, its advection to the slope edge in the bottom layer and descent down the slope. Special attention is payed to the role of coastal eddies arising due to a symmetric instability. These eddies promote the cross-shelf transport of the dense shelf water towards the continental slope edge. The simulation results are compared with rare observations of the deep slope convection in Peter the Great Bay. [ABSTRACT FROM AUTHOR]

Since the discovery of >570 methane flares on the northern U.S. Atlantic margin between Cape Hatteras and Georges Bank in the last decade, the acquisition of thousands of kilometers of additional water column imaging data has provided greater coverage at water depths between the outer continental shelf and the lower continental slope. The additional high-resolution data reveal >1400 gas flares, but the removal of probable duplicates from the combined database of new flares and those recognized in 2014 yields ∼1139 unique sites. Most of these sites occur in clusters of 5 or more seeps, leaving about 275 unique locations (including 47 clusters) for seepage along the margin. As a function of depth, seep distribution is heavily skewed toward the upper continental slope at water depths shallower than 400 m on the southern New England margin and ∼ 550 m in the Mid-Atlantic Bight, with additional seeps clustered at ∼1100 m and just deeper than ∼1400 m in both sectors. Despite little ongoing tectonic deformation or active faulting on this passive margin, a variety of processes driven from below the seafloor (e.g., migration of fluids along faults or through permeable strata, seepage above diapirs or other pre-existing structures) and from above (e.g., erosion, sapping, unroofing) contribute to the development of seeps in different settings along the margin. In addition, the prevalence of seeps on promontories overlooking shelf-breaking canyons may be directly related to the three-dimensional nature of the hydrate stability zone in these locations. As a function of depth, the parts of the slope at the contemporary landward limit of gas hydrate stability are devoid of seeps, and the upper slope zones with the most concentrated seepage were not within the gas hydrate stability zone even during the Last Glacial Maximum. Thus, if the large number of upper slope seeps is at least partially sourced in gas hydrate degradation, the gas emitted at these seeps must have migrated there from greater depths on the continental slope. • The number of known cold seep sites on the U.S. Atlantic margin has now doubled. • Seeps and clusters are found at ∼275 locations, mostly north of Cape Hatteras. • Most seeps are shallower than the landward limit of gas hydrate stability. • Faulting, erosion, unroofing, and hydrologic sapping dominate seep evolution. • Complex hydrate distribution leads to promontory seeps next to shelf-break canyons. [ABSTRACT FROM AUTHOR]

Carbonate mounds clustering in three fields were characterized on the upper continental slope of the northern Alboran Sea by means of a detailed analysis of the morphosedimentary and structural features using high-resolution bathymetry and parametric profiles. The contemporary and past benthic and demersal species were studied using ROV underwater imagery and some samples. A total of 325 mounds, with heights between 1 and 18 m, and 204 buried mounds were detected between 155 to 401 m water depth. Transparent facies characterize the mounds, which root on at least six erosive surfaces, indicating different growth stages. At present, these mounds are covered with soft sediments and typical bathyal sedimentary habitat-forming species, such as sea-pens, cerianthids and sabellid polychaetes. Nevertheless, remains of colonial scleractinians, rhodoliths and bivalves were detected and their role as potential mound-forming species is discussed. We hypothesized that the formation of these mounds could be related to favorable climatic conditions for cold-water corals, possibly during the late Pleistocene. The occurrence on top of some mounds of abundant rhodoliths suggests that some mounds were in the photic zone during minimum sea level and boreal guest fauna (e.g., Modiolus modiolus), which declined in the western Mediterranean after the Termination 1a of the Last Glacial (Late Pleistocene). [ABSTRACT FROM AUTHOR]

Benthic microbes are key organisms in the oligotrophic Southeastern Mediterranean Sea (SEMS), yet their abundance, activity, and diversity in this rapidly changing basin are not fully understood. We investigated the prokaryotic and microfungal communities throughout years 2018–2020 at 27 stations (6–1900 m water depths, down to 20 cm below the sediment surface), in two transects with distinct downslope transport regimes, and along the eutrophic coastline. We estimated microbial abundance with flow cytometry, secondary production as leucine assimilation, and sequenced marker genes (the 16S rRNA and internal transcribed spacer) to assess diversity indices. The highest abundance (0.21 × 108 cells gr–1 sediment) was estimated at slope stations where we assumed substantial transport rates and found an accumulation of organic carbon. Secondary production was the highest nearshore (12 ± 4 ng C gr–1 h–1), and markedly declined offshore (0.5 ± 0.9 ng C gr–1 h–1). Populations of archaea (dominant Nitrososphaeria and Nanoarchaeia) and diverse bacteria were stable over three years, and taxonomic composition was dictated mainly by depth gradients. Saprotrophic and pathotrophic microfungi Ascomycota (70% ± 23%) and Basidiomycota (16% ± 18%) were prevalent, whereas parasitic chytrids were abundant nearshore. Our results highlight the role of downslope transport, which enriched the typical deep-sea communities with anaerobic lineages, in shaping microbial populations near the continental slope. [ABSTRACT FROM AUTHOR]

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During the evolution of shrimp, essential aspects of their organization have remained unchanged. Approximately the same adaptations that have generated a large number of parallelisms within this relatively small group of arthropods have been used in the process of their adaptive evolution. This evolutionary process is typical telogenesis. The ancient shrimp fauna of both Dendrobranchiata and Pleocyemata may have originated in shallow waters not later than in the Jurassic. This is confirmed by the remains of Dendrobranchiata found in the shallow-water sediments of the Cenomanian (early Upper Cretaceous) and the remains of Pleocyemata as inhabitants of shallow-water refugia and representatives of the most primitive caridean shrimp family Procarididae. The last group is morphologically very similar to fossil shrimp of the Jurassic (for example, Udorella agassizi). Later, due to adaptive radiation at the generic level, shrimp occupied both the continental slope and the pelagic zone. These shallow-water species may have then been ousted by a new shrimp fauna during colonization of the shelf. The previous shrimp fauna may have been displaced to the continental slope, perhaps bordering on it. Some part of the species from various genera could have moved deeper along the slope down to the abyssal zone, whereas some others might have shifted to occupy the pelagic zone. This movement may have been associated with food resources, which were limited on the slope in the places with low levels of terrigenous sedimentogenesis. Numerous species seem to have switched from the detrital food chain to a grazing one. This process was repeated several times; as a result, the surviving representatives of the most ancient and subsequent fauna occurred at even greater depths. It follows from the above information that a pulsing type of telogenesis is characteristic of shrimp. Therefore, the modern shrimp fauna consists of young shelf groups with intensive cladogenesis, groups that have persisted since the Cretaceous (or even Jurassic), and remains of earlier fauna that have survived at great depths or in shallow-water refugia. [ABSTRACT FROM AUTHOR]

A Bottom Simulating Reflection (BSR) has been identified using 3D seismic data from the Cameroon continental slope margin. The BSR covers an area of c. 350 km2 in water depths ranging between 940 m to 1,750 m across an area characterized by high- and low-gradient slopes, gullies, scours, and fans. The thickness of the Gas Hydrate Stability Zone (GHSZ) is ~100 - 250 m, assuming an average velocity of 1,800 m/s. Pockmarks are intensively developed across the slope and most of them are observed in the BSR area. Geothermal estimation is based on hydrate stability conditions for pure methane - seawater system, hydrostatic pressure model, and a range of P-wave velocity models for the GHSZ, ranging from 1,600 -1,800 to 2,000 - 2,200 m/s. Geothermal gradient is calculated showing the range and distribution of thermal gradients in the BSR area from 0.046 oC/m to 0.094 oC/m with assumed GHSZ velocity of 1,800 m/s. Thermal gradient anomalies have been observed in association with gullies, vertically stacked channels and in some individual pockmarks. The highest anomalies of 0.08 oC/m - 0.094 oC/m are found in the depression areas of pockmark trains, within seafloor gullies. These positive anomalies are most likely controlled by active or recently active fluid advection and expulsion through the Cameroon slope. [ABSTRACT FROM AUTHOR]

Mega‐scale glacial lineations formed by the raking of ice shelves across the seafloor have been reported from multiple polar regions. Here, we present the first evidence of continental slope situated buried lineations in the southern Canadian Beaufort Sea in present‐day water depths of 220 to 800 m. Three separate surfaces with lineations are defined at sub‐seafloor depths of 40 m to 390 m. All lineations are mostly parallel to the general trend of slope contours. The uppermost surface is recognized over a distance of 56 km. In water depths > 500 m the lineations are parallel to each other at a consistent direction (43°–44°). The second lineated surface is a regionally occurring erosional unconformity. This event has two sub‐sets of lineations: mid‐slope situated lineations oriented at 42°–48°, and lineations closer to the continental shelf break at 55°–59°. The third lineated surface is an unconformable horizon buried up to 390 m below seafloor with lineaments oriented between 30° and 55°. All three sets of lineations are interpreted to have been produced by ice‐ploughing on the paleo‐seafloor through the grounding of an ice shelf. Our observations are similar to those documented along the slope off northern Alaska, Chukchi Rise, and Lomonosov Ridge. Collectively, these observations support the concept of an extensive ice shelf across the Arctic Ocean that grounded locally along its margins during multiple glaciations, including during the penultimate (or an earlier) glaciation. The youngest set of lineations indicates ice movement to the southwest with a suggested source in Amundsen Gulf and/or M'Clure Strait. Tentative age considerations for these youngest lineations indicate the first evidence for an analogous extensive ice shelf configuration for the Last Glacial Maximum. [ABSTRACT FROM AUTHOR]

Array gain is investigated based on the acoustic channel characteristics manifested by the fluctuant transmission loss and decrease in the acoustic channel spatial coherence. An analytical expression is derived as the summation of the products of the acoustic channel correlation coefficients and root-mean-square pressures. The formula provides insight into the physical mechanisms of the gain degradation in the ocean waveguide. Furthermore, this formula provides a new method to study array gain in the ocean waveguide from underwater acoustic field. The obtained expression is a more general formula that is applicable to shallow water, deep sea, and continental slope, with the traditional methods as a special case. Numerical results show that the array gain calculated by previous formulas are generally overestimated, caused by ignoring the effect of transmission loss fluctuation. [ABSTRACT FROM AUTHOR]

The fauna of benthic foraminifers from the northern Laptev Sea has been analyzed to identify the relationship between the distribution of benthic foraminifers and environmental conditions in the eastern and western parts of the sea. The abundance of benthic foraminifer tests in the Laptev Sea is universally low, no more than 3–9 ind./g. Greater foraminifer abundances were found in the west, on the shelf of the Vilkitsky Strait. Foraminifers with agglutinated tests were dominant, 77–100%. In the east, the assemblages consisted almost entirely of agglutinated tests (90–100%). Calcareous Melonis barleeanus was common, up to 10–15%, on the shelf and the continental slope in the western part of the sea. Small specimens of the polar planktic species Neogloboquadrina pachyderma sin occurred. In one sample at an abyssal water depth of 2996 m, the abundance of its tests increased to 302 ind./g; the assemblage was composed predominantly of N. pachyderma sin., both small (juvenile) and large (adult) individuals with good preservation. The massive presence of the polar species N. pachyderma sin. at an abyssal depth in the eastern region indicates the dominant influence of Arctic waters there. [ABSTRACT FROM AUTHOR]

Internal solitary waves (ISWs) can cause strong seafloor sediment resuspension and induce nepheloid layers in both shallow and deep-water environments. However, the roles of ISWs in the >1000 m deep sea sediment resuspension and seafloor geomorphic changes are still unclear. To answer the above question, in the Dongsha area of the northern South China Sea, we measured suspended particulate matter along with a section covering the entire continental slope between 300 and 2000 m water depths, together with high-resolution multibeam bathymetric data for examining geomorphic changes. The results indicate that, on the upper slope with water depth <700 m, seafloor sediments were heavily disturbed and resuspended. We find that ISWs could suspend seabed sediments and shape a bedform at water depths ≲1000 m. The maximum water depth of sediment resuspension by ISWs measured is found as deep as 1500 m. The distribution pattern of the seafloor surface sediments on the east of the Dongsha continental slope (fine and silty sand in <700 m water depth, clayey silt between 700 and 1500 m, and silty clay > 1500 m) also indicates that they are mainly controlled and impacted by ISWs. The wave refraction theory could be applied to the upper slope, but sediment resuspension is related to the seabed topography on the lower slope. Our study shows that the suspension and transport of sediments induced by episodic ISWs on the Dongsha slope of the northern South China Sea could shape the bedform and affect the sedimentary seabed geomorphology. This research will help explain the impacts of the ISWs on the deep-water sediment resuspension and seafloor geopmorphic changes along with the continental slope in the margin sea. [ABSTRACT FROM AUTHOR]

Living (rose Bengal stained) benthic foraminiferal faunas, grain size, geochemical parameters (total organic matter, chlorophyll-a, pheophytin-a, molecular markers, and vertical flux of particulate organic matter) were investigated in 14 stations in two submarine canyons Almirante Câmara and Grussaí considered mature and immature, respectively, and on the adjacent continental slope, from 400 to 1300 m water depth, in the southwestern Atlantic Ocean. Samples were retrieved during two campaigns, in the austral winter 2008 and the austral summer 2009. The density, richness, and species composition of the benthic foraminiferal fauna decrease with depth in the canyons and along the adjacent slope transects, reflecting the availability and quality of sedimentary organic matter. Both canyons act as traps of labile organic matter, mainly towards the head of the Almirante Câmara canyon, promoting high specimen density, species richness, and the occurrence of species indicative of pulses of phytodetritus (e.g., Bolivina pacifica , Nonionella opima , and Nonionella stella). Towards the deeper regions of Almirante Câmara canyon and in the adjacent slope area, the lower abundance, diversity, and richness reflect more oligotrophic conditions and an adverse environment in terms of easily metabolizable organic matter. This multiproxy study demonstrates that the canyons in the southeastern Brazilian continental margin play an important role in accumulating organic matter. • Response of benthic foraminifera for particulate organic matter flux. • Behavior of benthic foraminifera to organic compounds in canyons. • Canyons as traps of organic matter in an oligotrophic area. • Colonization by benthic foraminifera after events of sediment disturbance. [ABSTRACT FROM AUTHOR]

The adaptive array invariant developed for source-range estimation in shallow water can incorporate the propagation-angle dependence of the waveguide invariant for an ideal waveguide (β = cos 2 θ) [Byun and Song, J. Acoust. Soc. Am. 148, 925–933 (2020)]. This paper extends the approach to weakly (adiabatic) range-dependent environments with variable bathymetry, wherein the waveguide invariant is a complex function of the bathymetry between source and receiver as well as the propagation angle in powers of sin 2 θ. For a given bathymetry, the adaptive array invariant can be implemented in an iterative fashion, and its remarkable performance is demonstrated using a short-aperture vertical array (2.8 m) for a broadband source (0.5–3.5 kHz) towed on a continental slope where the water depth varies from 87.5 to 55 m over a 5-km range. [ABSTRACT FROM AUTHOR]

Three new species of Microspio were found in collections from a range of depths including shallow water in Bermuda, the continental shelf off Massachusetts, and deep water in the South China Sea off Brunei. Microspio fischeri, new species, is associated with shallow water sponges in Bermuda where it is a prey species of a predatory syllid; it is characterized by the medial placement of the first pair of branchiae as well as unusual notopodial capillaries in anterior chaetigers and enlarged notopodial lobes in posterior chaetigers. Microspio lydonia, new species, from the continental shelf on the east coast of the United States is distinguished from M. pigmentata (Reish, 1959) from the west coast by differences in the structure of the nuchal organs and hooded hooks, pigmentation, and Methyl Green staining patterns. Additions to the description of M. pigmentata include illustration of the nuchal organs and dorsal ciliary bands. Microspio ariena, new species, from deep water in the South China Sea off Brunei has enlarged branchiae with large glands that stain deeply with Methyl Green on a few anterior chaetigers, followed by much smaller branchiae on additional chaetigers. The three new species bring the total of described species in the genus to twenty-one, although some species remain problematic. A table comparing important characteristics of all 21 known species of Microspio is included. [ABSTRACT FROM AUTHOR]

This study focuses on gas bubble emissions across ~ 1800 km2 of the southeastern continental slope of the Crimean Peninsula in the Eastern Black Sea basin. In total, about 3500 gas emissions were mapped as so-called flares in single- and multibeam echosounder imaginary. Flares generally emanate from the seafloor, with a few exceptions, where water depths are less than 700 m, which represents the upper limit of the gas hydrate stability zone in this region. Methane in shallow sediments below 700 m is transferred to methane hydrate, and the migration of free gas into the water column is therefore inhibited in most cases. This study illustrates further that flare occurrence is linked to morpho-structural margin characteristics. Two morphologically different margin segments are distinguished, and flare distributions were found to relate to differing morphological structures along these segments. In the western province, where the upper continental margin is steep and incised by gullies, flares occur mainly along downslope-oriented ridge crests. In the eastern province, representing a part of the Don-Kuban paleo-fan, the slope is influenced by extensive mass wasting. Gas emissions in this area were mainly detected aligned along scarps of submarine landslides and related to sediment failures. Almost all flares disappear in the acoustic records of the water column below a water depth of ~ 100 m suggesting that the gas bubbles dissolve within the water body of the Black Sea and contribute to its methane inventory and the anoxic water column. [ABSTRACT FROM AUTHOR]

The Lagrangian characteristics of the surface flow field arising when an idealized, anticyclonic, mesoscale, isolated deep-ocean eddy collides with continental slope and shelf topography are explored. In addition to fluid parcel trajectories, we consider the trajectories of biological organisms that are able to navigate and swim, and for which shallow water is a destination. Of particular interest is the movement of organisms initially located in the offshore eddy, the manner in which the eddy influences the ability of the organisms to reach the shelf break, and the spatial and temporal distributions of organisms that do so. For nonswimmers or very slow swimmers, the organisms arrive at the shelf break in distinct pulses, with different pulses occurring at different locations along the shelf break. This phenomenon is closely related to the episodic formation of trailing vortices that are formed after the eddy collides with the continental slope, turns, and travels parallel to the coast. Analysis based on finite-time Lyapunov exponents reveals initial locations of all successful trajectories reaching the shoreline, and provides maps of the transport pathways showing that much of the crossshelf- break transport occurs in the lee of the eddy as it moves parallel to the shore. The same analysis shows that the onshore transport is interrupted after a trailing vortex detaches. As the swimming speeds are increased, the organisms are influenced less by the eddy and tend to show up en mass and in a single pulse. [ABSTRACT FROM AUTHOR]

The Solonker Belt is an important tectonic unit of the Central Asian Orogenic Belt in Inner Mongolia, China and the sediments in the Solonker Belt record the paleogeographic evolution of the Paleo-Asian Ocean. This study characterizes the spatial distribution of the lithofacies and lithofacies associations, and environment variations in the Mandula Basin. The Upper Carboniferous to Lower Permian Amushan Formation shows typical features of slump deposits occurred in the lower slope environments. The Lower to Middle Permian Baotege Formation develops the channelized lobes, channel–levee–overbank, and hemipelagic/pelagic deposits that are formed in the base of slope to basin floor environments. The Lower to Middle Permian Dashizhai Formation represents volcanic edifice and volcaniclastic aprons in the basin floor. The Middle Permian Zhesi Formation includes the shallow water reefs, deep-water biogenetic limestones, and volcaniclastic aprons developed in the abyssal slopes of the volcanic edifice. The sedimentary facies in the Mandula Basin constitute two depositional systems: the continental slopes adjacent to marginal carbonate platform and the volcaniclastic aprons associated with volcanic edifice in the basin floor. The Mandula Basin is an extensional deep-water basin and akin to the marginal sea of an ocean. [ABSTRACT FROM AUTHOR]

The major (Mw 7.9) earthquake that struck the Gulf of Alaska near Kodiak Island on 23 January 2018 was a rare, mid-plate strike-slip event that triggered a minor trans-Pacific tsunami. An analysis of the simultaneous measurements of tsunami waveforms at 21 open-ocean sites (including three independent arrays of stations) and 27 coastal tide gauges in the Gulf of Alaska and along the coast of North America has enabled us to examine properties of the 2018 tsunami, its transformation over the continental slope and shelf, and its amplification as the waves approached the coast. Results show that the tsunami wave variance decreased monotonically along the west coast from northern British Columbia to southern Oregon. Based on the variance structure, the mean amplification factor for Tofino on the west coast of Vancouver Island (a "beacon" site with a long time series), was A RMS Tof = 5.3, in good agreement with corresponding estimates for four major past events; 4.5 (2009 Samoa), 4.3 (2010 Chile), 6.3 (2011 Tohoku) and 5.2 (2012 Haida Gwaii). This variance-derived amplification for Tofino was greater than the amplification factor based on the amplitude ratio ( A Tof = 3.2). Spectral analysis of the records showed that the tsunami had a relatively large high-frequency content (i.e., was "blueish"), with nearly 90% of the total energy in the open ocean at frequencies > 1.7 cph (periods < 35 min) and with an "integral frequency scale" of 4 cph (period 15 min). Wavelet analysis revealed strong dispersion of the propagating tsunami waves, in agreement with theoretical estimates. The abrupt jump in water depth of about 4 cm detected at DART 46409, located mid-plate about 85 km from the epicenter of the 2018 Kodiak earthquake, appears to have been due to an earthquake-induced seafloor subsidence. [ABSTRACT FROM AUTHOR]

The propagation of long waves onto a continental shelf is of great interest in tsunami modeling and other applications where understanding the amplification of waves during shoaling is important. When the linearized shallow water equations are solved with the continental shelf modeled as a sharp discontinuity, the ratio of the amplitudes is given by the transmission coefficient. On the other hand, when the slope is very broad relative to the wavelength of the incoming wave, then amplification is governed by Green's Law, which predicts a larger amplification than the transmission coefficient, and a much smaller amplitude reflection than given by the reflection coefficient of a sharp interface. We explore the relation between these results and elucidate the behavior in the intermediate case of a very steep continental shelf. [ABSTRACT FROM AUTHOR]

230Th normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of 230Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230Th from across the global ocean at two time slices, the late Holocene (0–5,000 years ago, or 0–5 ka) and the Last Glacial Maximum (18.5–23.5 ka), and investigated the spatial structure of 230Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79–2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48–1.68 g/cm2kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size‐dependent sediment fractionation, and carbonate dissolution on the efficacy of 230Th as a constant flux proxy. Anomalous 230Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that 230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth). Key Points: 230Th normalization is a robust tool for calculating sedimentary mass fluxes230Th may be affected by hydrothermal and boundary scavenging in certain discrete regionsGenerally, 230Th mass fluxes are preferable over age model‐based mass accumulation rates [ABSTRACT FROM AUTHOR]

Risso's dolphin occurs in tropical and temperate waters worldwide, and is one of the most common cetacean species in Taiwanese waters. This study attempted to reveal its distribution in water-depth ranges and variations in group size and composition (with or without mother-calf pairs). Based on sighting records from experienced whale-watching guides on board whale-watching boats operated in eastern Taiwan, 1630 sightings of Risso's dolphin were recorded from 3939 touring trips near Hualien County from 1998 to 2014. Comparing the frequency distribution of water depths between ship tracks and dolphin sightings, we found that Risso's dolphin commonly occurred on the continental shelf and slope, and mainly stayed in waters of 300~1500 m in depth, with a mean of 862.1 ± 12.3 m (standard error). The frequency distribution of water depths showed a significant difference between sites (deeper in the Shirti area than in the Hualien area), but no significant difference was detected among various group sizes. Nevertheless, the spatial distribution significantly varied with group composition, but not with group size. Groups with mother-calf pairs stayed in significantly shallower water, which was proposed to be related to predator avoidance. In addition, the large fluctuation in annual sighting ratios of Risso's dolphin that ranged 0.1~0.7 sightings per trip during the 17-year period may be related to variations in prey abundances, the confirmation of which requires future investigation. [ABSTRACT FROM AUTHOR]

A hazard assessment of submarine landslide-generated tsunami for the east Australian continental slope between Jervis Bay and Fraser Island is presented. Submarine landslides are present in water depths of approximately 400 to 3500 m along the entire length of continental margin, but are increasingly prevalent northward of Coffs Harbour without clustering at any particular water depth. Two hundred sixty individual submarine landslide scars that are greater than 1 km in width have been identified. Of these, 36 have been calculated to produce a tsunami flow depth equal to or greater than 5 m at the coastline for an assumed landslide downslope velocity of 20 ms−1. Landslides that are both thick (> 100 m) and wide (> 5 km) have the greatest potential to generate the largest coastal flow depths (> 10 m). The water depth of a landslide's centre of mass strongly influences the onshore height of the tsunami's surge with the larger events generated in shallower water depths (~ 500–1500 m). The maximum flow depth at the coastline is larger for thicker (50–250+ m) canyon landslides which occur on steeper slopes (> 4°), compared to thinner (< 50 m) plateau landslides which generally produce smaller tsunami. Maximum inundation distances and run-up heights of 1.6 km and 22 m respectively have been calculated for landslide velocities of 20 ms−1 and these values vary significantly depending on local coastal topography. There is no evidence for a submarine landslide large enough and young enough to have generated a Holocene age megatsunami for the east coast of Australia. [ABSTRACT FROM AUTHOR]

Bottom trawling in the deep sea is one of the main drivers of sediment resuspension, eroding the seafloor and altering the content and composition of sedimentary organic matter (OM). The physical and biogeochemical impacts of bottom trawling were studied on the continental slope of the Gulf of Castellammare, Sicily (southwestern Mediterranean), through the analysis of two triplicate sediment cores collected at trawled and untrawled sites (∼550 m water depth) during the summer of 2016. Geochemical and sedimentological parameters (excess 210Pb , excess 234Th , 137Cs , dry bulk density, and grain size), elemental (organic carbon and nitrogen) and biochemical composition of sedimentary OM (proteins, carbohydrates, lipids), as well as its freshness (phytopigments) and degradation rates were determined in both coring locations. The untrawled site had a sedimentation rate of 0.15 cm yr -1 and presented a 6 cm thick surface mixed layer that contained siltier sediment with low excess 210Pb concentrations, possibly resulting from the resuspension, posterior advection, and eventual deposition of coarser and older sediment from adjacent trawling grounds. In contrast, the trawled site was eroded and presented compacted century-old sediment highly depleted in OM components, which were between 20 % and 60 % lower than those in the untrawled site. However, the upper 2 cm of the trawled site consisted of recently accumulated sediments enriched in excess 234Th , excess 210Pb , and phytopigments, while OM contents were similar to those from the untrawled core. This fresh sediment supported protein turnover rates of 0.025 d -1 , which doubled those quantified in surface sediments of the untrawled site. The enhancement of remineralization rates in surface sediment of the trawled site was associated with the arrival of fresh particles on a chronically trawled deep-sea region that is generally deprived of OM. We conclude that the detrimental effects of bottom trawling can be temporarily and partially abated by the arrival of fresh and nutritionally rich OM, which stimulate the response of benthic communities. However, these ephemeral deposits are likely to be swiftly eroded due to the high trawling frequency over fishing grounds, highlighting the importance of establishing science-based management strategies to mitigate the impacts of bottom trawling. [ABSTRACT FROM AUTHOR]

The temporal evolution of water column turbidity was studied on a submarine canyon on the Barcelona continental margin. From April to June 2014, an instrumented mooring array equipped with an autonomous hydrographic profiler with a CTD and a turbidimeter was deployed in the Foix canyon axis at 870 m depth. The instruments were programmed to collect hydrographic profiles once per day to provide a view of the temporal evolution of water column characteristics from 200 to 800 m water depth. The results illustrate a well-defined water turbidity structure of particulate matter distributed in intermediate nepheloid layers (INLs) developed between 300 and 500 m water depth and above the canyon rims, and INLs and near-bottom nepheloid layers (BNLs) confined inside the canyon between 650 and 800 m water depth. Data from fishing vessels activity at the time of the deployment was obtained from Vessel Monitoring System (VMS). The presence and location of the fishing vessels in the study area suggested a relationship between trawling activity and the generation of such layers. Nepheloid layers were absent during the first part of the deployment, when there was no fishing activity within the Foix Canyon axis or at the adjacent continental slope. Later, with the beginning of trawling activity on the fishing grounds close to the canyon, both INLs and BNLs were observed in the profiling casts, suggesting a causative relationship with fishing activities. Additionally, the hydrodynamic conditions within the canyon also seem to favour particle retention and to increase water turbidity in thick BNLs when water circulation is directed up-canyon. Bottom trawling appears to act as a main sediment resuspension mechanism in the Barcelona continental slope regions, increasing suspended sediment concentration at specific water depths where fishing grounds are located. Suspended particles are then advected and propagate along and across-margin by ambient currents via nepheloid layers. • Particulate matter in the Foix Canyon was mainly distributed in INLs and near-BNLs. • INLs and BNLs were observed when fishing activity was detected close to the canyon. • Hydrodynamic conditions seem to favour particle retention and to increase turbidity. • Bottom trawling acts as a main sediment resuspension mechanism in the Barcelona margin. [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge could impact the transport of critical solutes to the ocean. However, its driver(s), significance over geological time scales, and geographical coverage are poorly understood. We characterize a submarine groundwater seep from the continental slope off northern Norway where substantial amount of meteoric water was detected. We reconstruct the seepage history from textural relationships and U‐Th geochronology of authigenic minerals. We demonstrate how glacial‐interglacial dynamics have promoted submarine groundwater circulation more than 100 km offshore and result in high fluxes of critical solutes to the ocean. Such cryosphere‐hydrosphere coupling is likely common in the circum‐Arctic implying that future decay of glaciers and permafrost in a warming Arctic is expected to attenuate such a coupled process and thus decreases the export of critical solutes. Plain Language Summary: Occurrence of meteoric groundwater (freshwater originated from precipitation such as rain and snow) in the global ocean is an unexpected but seemingly common phenomenon. Here, we report evidence for meteoric groundwater flow at ~800‐m water depth from the coast of northern Norway. Dating of the chemically formed carbonate rocks on the seafloor, an oxidation product of methane, reveals that the groundwater flow was strongest when large ice sheets occupied the nearby shelf. Our results confirm the temporal and geographical scales of meteoric groundwater flow in the Arctic region and highlight its impact on carbon cycling and ocean chemistry. Key Points: We document the connection between glacial dynamics and the discharge of meteoric water from the eastern Norwegian SeaA large quantity of critical solutes discharge to the ocean as a result of water circulationOffshore presence/discharge of freshwater is a common phenomenon in the circum‐Arctic Ocean that have profound impact on ocean chemistry [ABSTRACT FROM AUTHOR]

Bottom trawling in the deep sea is one of the main drivers of sediment resuspension, eroding the deep seafloor and altering the content and composition of sedimentary organic matter (OM). The physical and biogeochemical impacts of bottom trawling on the seafloor were studied in the continental slope of the Gulf of Castellammare, Sicily (Southwestern Mediterranean) through the analysis of two triplicate sediment cores collected in trawled and untrawled sites (~ 550 m water depth) during the summer of 2016. Geochemical and sedimentological parameters (excess 210Pb, excess 234Th, 137Cs, dry bulk density, and grain size), elemental (organic carbon and nitrogen) and biochemical composition of sedimentary OM (proteins, carbohydrates, lipids), as well as its freshness (phytopigments) and degradation rates were determined in both coring locations. The untrawled site had a sedimentation rate of 0.15 cm yr−1 and presented a 6-cm thick surface mixed layer that contained coarser sediment with low excess 210Pb concentrations, possibly resulting from the resuspension, posterior advection, and eventual deposition of siltier and older sediment from adjacent trawling grounds. In contrast, the trawled site was characterized by highly eroded and compacted century-old sediment, as shown by the lack of excess 210Pb and high dry bulk densities. The continuous erosion in the trawled site has led to the depletion of OM, which were between 20 % and 60 % lower than those in the untrawled site, as well as to statistically significant differences in the biochemical composition of OM. Nevertheless, the upper 2 cm of the trawled site consisted of recently accumulated sediments, enriched in excess 234Th, excess 210Pb, and phytopigments, which had similar OM contents to surface sediments from the untrawled core. The arrival of fresh sediment in a chronically-trawled deep-sea site that is generally deprived of OM was associated with an enhancement of remineralization rates, reflected by protein turnover rates of 0.025 d−1, which doubled the rates quantified in surface sediments of the untrawled site. We conclude that the detrimental effects of bottom trawling can be temporarily and partially abated by the arrival of fresh and nutritionally-rich OM, which stimulate the response of benthic communities. However, these ephemeral deposits are likely to be swiftly eroded due to the high trawling frequency over fishing grounds, highlighting the importance of establishing management strategies to mitigate the impacts of bottom trawling. [ABSTRACT FROM AUTHOR]

Most sequence stratigraphy models have been derived from the observation of seismic profiles, which still retain the original form of the sedimentary stage. For folded and fractured formations, there is no way to establish a full view of the basin-wide continuous stratal configuration. This paper presents an approach to restore the sedimentary sequence original structural profile through a process of segmented line-drawing of seismic events, reassembly and modification. We use a case study from the Pearl River Mouth Basin, in the South China Sea, to demonstrate the principle and method of this technology. After restoration, we obtained a basin-wide, sedimentary stage form and continuous sequence stratigraphic structure profiles, and thus further investigated the sedimentary landforms, palaeo-water depth, sedimentary systems, stacking pattern, stratal configuration, parasequence sets and systems tract, which may be ambiguous in conventional seismic profiles. This approach may be applied to the study of basin and sequence stratigraphy and basin sedimentary process. • A new technique for restoring the sedimentary sequence original form seismic profile by the method of line-drawing. • Remove faults and folds, a complete sequence stratigraphic profile of the entire basin can be displayed on A4 paper. • The original form profile can be used to determine the genetic unit (systems tract) and sedimentary environment. • To estimate the water depth and determine the source rock (condensed section) buried in deep depth. [ABSTRACT FROM AUTHOR]

Abstract In the northern South China Sea, numerous multichannel seismic reflection sections are used to identify internal solitary wave (ISW) packets and extract wave amplitudes and corresponding water depths. The analyzed data show that these depression ISWs occur on the upper continental slope at water depths between 263 m and 740 m, with maximum amplitudes ranging from 35 m to 128 m. Our results, in conjunction with previous studies, suggest that the maximum amplitudes of the ISWs on the northern South China Sea continental slope are highly correlated with seafloor depths and that they have a logarithmic function relationship. The maximum amplitudes decrease with decreasing water depths. Interactions between the ISWs and the seafloor play a crucial role in decreasing ISW maximum amplitudes, especially in shallow areas. In addition, we compare the observed vertical amplitude distribution with theoretical results and find that they are concordant. The "bottom depth" (H) in the boundary conditions of eigenfunctions represents the extension depth of the ISW rather than the actual seafloor depth. Here, the ISW extension depth is where the ISW amplitude becomes zero and the seafloor depth is just under the ISW. If the ISW interacts intensely with the seafloor, its observed vertical amplitude distribution may exhibit prominent differences from the theoretical result. Highlights • Fourteen individual internal solitary waves have been identified in the northern South China Sea from multi-channel seismic data.. • A logarithmic correlation between maximum amplitude and corresponding depth has been observed. • The individual ISW's amplitude distribution over depth can be best described by the solution of eigenfunctions. [ABSTRACT FROM AUTHOR]

Abstract Understanding community assembly and processes driving diversity in deep-sea environments is a major challenge in marine ecosystems. In this paper, we investigated the importance of environmental gradients at different spatial scales in structuring deep-sea canyon and continental slope meiobenthic nematode communities in the NW Mediterranean Sea. Three scales were investigated 1) Ecosystem: Blanes Canyon vs. adjacent open slope; 2) Water-depth within each ecosystem: 1500, 1750 and 2000 m; and 3) Vertical profile (three layers within the first 5 cm of sediment). Nematode communities were analysed in terms of density, biomass, diversity and community structure. Grain size, Chl a , Chl a : phaeopigments, CPE, organic carbon and total nitrogen were measured to assess the relationships with nematode assemblages. Blanes Canyon harbours more abundant and diverse assemblages than the slope, particularly at 1750 m and 2000 m depth respectively. The higher canyon values may be related to the higher food availability there, which can be a consequence of the so-called "canyon effect". Slope assemblages were overall more uniform than those in the canyon, where there were greater bathymetrical differences in community structure. Densities in the canyon peaked at 1750 m depth, which did not correspond with the bathymetric gradient in food availability. The deepest canyon station was the most similar to the slope stations in terms of both environmental conditions and nematode communities, suggesting it lie outside the canyon influence. The higher habitat heterogeneity of the canyon (indicated by its greater vertical sediment and water depth differences) played a key role in structuring nematode spatial distribution. Independently of the ecosystem, however, the vertical sediment profile was the most determinative factor for density, community structure, and diversity. Highlights • Nematode community (standing stocks, diversity and community structure) differed between canyon and slope. • Blanes Canyon harbours more spatially heterogeneous and diverse nematode communities than its adjacent open slope. • Vertical sediment profile was significantly affecting the community structure and diversity, both in canyon and slope. [ABSTRACT FROM AUTHOR]

Abstract The NW Iberian continental margin is characterised by a complex morphology and by a sedimentary system which was highly dynamic over glacial to interglacial times. The sedimentary history of the continental slope was strongly influenced by the interaction of bottom currents with topographic highs of structural origin leading to the accumulation of several sediment drifts. A combined analysis of gravity cores from different water depth with hydroacoustic data reveals the vertical behaviour of the upper Mediterranean Outflow Water (MOW) core after the Last Glacial Maximum (LGM). A coarser grained interval during Deglacial and early Holocene times (17.2 to 9.9 cal ka BP) points to an increase in bottom current strength. This increase in velocity was probably related to oceanic density fronts, which migrated through the 300 m thick transition zone between the underlying Labrador Sea Water and the overlying MOW. Radiocarbon dates timed the current strengthening to 17.2 cal ka BP, and a following weakening of the bottom current to 13.3 cal ka BP at 1965 m water depth and to 9.9 cal ka BP at 1885 m water depth. The depth-dependent current weakening suggests an upward shifting of the transition zone by 80 m that was related either to an overall shallowing of MOW or a vertical contraction of this water mass. The upward movement happened over a time interval of approximately 3.4 thousand years. In addition sediment core analysis reveals significant lateral heterogeneities within cm to dm thick sediment layers in the contourite drift. These heterogeneities suggest a need of a detailed core coverage across current-influenced deposits for palaeoceanographic studies to minimize misinterpretations. Highlights • A contourite drift records bottom current strengthening during deglacial times. • The sedimentary succession documents an upward shift of the Mediterranean Outflow Water 16 during the late deglacial times. • The upward shift of the Mediterranean Outflow Water lower boundary by 80 m occurred after 13.3 cal ka BP over about 3,4 ka. • The Si/Al element ratio provides a semi-quantitative proxy for grain size variations at the NW 20 Iberian margin. • Contourite drift investigated represents internal lateral heterogeneous sediment pattern. [ABSTRACT FROM AUTHOR]

Large sediment waves have been observed over the Gulf of Roses (GoR) continental slope (NW Mediterranean), developed between ~200 and ~400 m water depth. Geometric parameters computed from the acquired swath bathymetry revealed mean wave lengths of ~2000 m, and maximum wave heights of ~60 m. Single-channel reflection seismic profiles provided information on the sediment wave internal structure and the Quaternary stratigraphic architecture of the GoR outer shelf and slope. Seven main seismic units could be identified, with continuous development of sediment waves over the outer continental shelf and upper slope, showing differences in wave height and length. The seismic units are differentiated by erosional surfaces that can be followed from the outer shelf down the slope, and which have been correlated with Pleistocene eustatic oscillations. Sediment cores were collected over the sediment wave crests and troughs, and grain size distribution and sediment accumulation rates were analysed. Results show a dominant fraction of fine sediments, allowing classifying the observed bedforms as mud waves. Calculated sediment accumulation rates ranged between 0.08 and 0.18 cm/y, with no clear sedimentation pattern (e.g. differential sediment deposition rates) observed between wave crests and troughs. Nevertheless, the presence of thick surface mixed layers and the increase of the sand fraction in the upper sections of the cores indicate that the surface sediments are affected by bottom trawling activities, since the area is highly impacted by this human activity. The sediment waves observed over the GoR slope are most likely to be formed by bottom currents generated by overflows of dense water originated in the Gulf of Lions shelf, which cascade downslope in an oblique angle with respect the main bathymetric contours. This study offers new insights on the role of dense shelf water cascading processes and associated off-shelf sediment transport reshaping the morphology of the open-slope regions. [ABSTRACT FROM AUTHOR]

This study explores the first record of spatiotemporal distributions of macro and micro-litter on the seafloor in the Southeastern (SE) Levantine Basin (LB) during 2012–2021. Macro-litter was surveyed by bottom trawls in water depths of 20-1600 m and micro-litter by sediment box corer/grab at a depths range of 4-1950 m. Maximal macro-litter concentrations were recorded at the upper continental slope (200 m), averaging 4700 ± 3000 items/km2. Plastic bags and packages were the most abundant items (77 ± 9 %) with a maximum of 89 % at 200 m depth, and their size decreased with increasing water depth. Micro-litter debris were found mainly in shelf sediments (≤30 m water depth) with an average concentration of 40 ± 50 items/kg, while shit particles transferred to the deep sea. These findings suggest an extensive distribution of plastic bags and packages in the SE LB, predominantly accumulating in the upper continental slope and deeper, based on their size. • Spatiotemporal distributions of macro & micro-litter were studied for the 1st time in the SE Levantine Basin. • Maximal macro-litter concentrations were recorded at the upper continental slope. • Plastic bags and packages were the most abundant items, and their size decreased with increasing water depth. [ABSTRACT FROM AUTHOR]

Near-inertial waves (NIWs) play an important role in determining oceanic energy and are highly related to oceanic dynamics, including tropical cyclones (TCs). However, most studies have focused on the features of NIWs in the upper layer, and the characteristics of NIWs near the seabed remain poorly understood. Based on current meter records from two moorings, we examined the characteristics of NIWs during the passage of four TCs, namely, Fengshen, Kammuri, Nuri and Hagupit, in the northern South China Sea (SCS). The two moorings were deployed on the continental slope, with one located in relatively shallow water, i.e., the top of the slope, and the other in relatively deeper water, i.e., in the middle of the slope. The data from the shallower mooring revealed that the near-inertial kinetic energy (NIKE) near the seabed was significantly enhanced during the passage of TCs Nuri and Hagupit. During the passage of these TCs, an elongated near-inertial current (NIC) ellipse was observed near the seabed with the major axis aligned perpendicular to the isobaths; these NICs were different from the slightly elliptical NICs observed at the deeper mooring. Considering the topographic slope around the shallower mooring, the NICs with different properties were caused by the reflection of NIWs near the seabed. The Richardson number decreased to below 0.25 during TC Hagupit, indicating that shear instability tended to occur near the seabed at the shallower mooring when enhanced NICs were superimposed on subinertial currents. A scaling analysis indicated that the downward penetration of TC-generated NIWs was partly blocked by the shear of subinertial currents during TC Fengshen. This partly explains why most of the NIKE was concentrated in the upper layer during TC Fengshen. This study, based on observations, highlights that the elongated NIC ellipse and strong shears of NIWs that occurred near the seabed were caused by NIW reflection. Therefore, part of the TC-generated NIKE was reflected by the seabed, and part of the NIKE might be dissipated due to shear instability. • Near-inertial currents (NICs) near the seabed were significantly enhanced during the TCs Nuri and Hagupit. • The NICs ellipse became elongated, with major axis aligned perpendicular to isobaths near the seabed. • Strong shears occurred when near-inertial waves collided with the seabed and shear instability might happen. [ABSTRACT FROM AUTHOR]

The Kuroshio Current, originating from the West Pacific Warm Pool and flowing through the Okinawa Trough, connects the equator and the subtropical West Pacific and plays an important role in the carbon cycle in the West Pacific Ocean. Previous studies have focused on the effects of surface export productivity and bottom redox conditions on the carbon cycle in the late Quaternary. Recent studies have demonstrated that methane seepages are very common in the Okinawa Trough, however, their significance for the carbon cycle remains not well understood. Here, methane seepages and sulfate-driven anaerobic oxidation of methane (SD-AOM) are constrained by the geochemical characteristics of porewater at site CSHC-4 (Length: 1555 cm) derived from the water depth of 934 m on the western continental slope of the Okinawa Trough. Using the △(DIC+ Ca2++ Mg2+)/△SO 4 2− ratio, it is suggested that the sulfate reduction in the upper part of site CSHC-4 is dominated by organoclastic sulfate reduction (OSR), while that between 655 and 1055 cm is gradually dominated by SD-AOM with deepening depth. However, with the depletion of sulfate, the methanogenesis process predominates at the bottom of site CSHC-4. The depth of the sulfate-methane interface (SMI) and the methane diffusive flux of site CSHC-4 is calculated to be ∼1156 cm and ∼ 63 mmol m−2 yr−1, respectively. Near the SMI, the changes in concentrations of Ca2+, Mg2+, Sr2+ and ratios of Mg/Ca and Sr/Ca in the porewater indicate the possibility of the precipitation of authigenic high-Mg calcite in the sediments. In contrast, the increase in the concentration of Ba2+ in porewater below the SMI may imply the dissolution of barite (BaSO 4). In addition, redox-sensitive elements in porewater exhibit various geochemical behaviours due to changes in redox conditions caused by SD-AOM. The above SD-AOM processes will affect the reconstruction of surface productivity and redox conditions of bottom water in the Okinawa Trough since the late Quaternary based on biogenic elements (e.g., Ca and Ba) and redox-sensitive elements (e.g., U and Cr), which requires careful evaluation. Therefore, our new findings illustrate the modern methane seepage in the Okinawa Trough and provide a reference for calculating the ancient carbon cycle in the subtropical West Pacific. • The methane diffusive flux is calculated to be ∼63 mmol m−2 yr−1 at site CSHC-4. • The depth of the sulfate-methane interface (SMI) is located at approximate1156 cm. • SD-AOM led to precipitation of carbonate minerals and dissolution of barite in sediments. • The SD-AOM process affects the evaluation of history productivity and redox conditions. [ABSTRACT FROM AUTHOR]

The importance of internal solitary waves (ISWs) to the energy and material transport in the upper ocean has been confirmed, but how ISWs affect turbulent mixing in the bottom boundary layer is lack of direct field observation. To reveal the characteristics of ISWs and the turbulent mixing they cause in the bottom boundary layer, a 36-h fine-scale observation of ISWs and their influence on turbulent mixing was conducted at the South China Sea slope (water depth: 659 m). During the observation, a group of ISWs passed by with horizontal and vertical velocities of 0.2 and 0.04 m/s at 0.1 m above bottom (mab) respectively, resulting in strong velocity shear exceeding 0.5 m/s per meter. When the ISWs passed through the slope, although they did not deform and break near the seabed, they increased the bottom shear stress, turbulent kinetic energy production rate and turbulent kinetic energy dissipation rate (ε) at 0.6 mab several times. The ISWs induced ε reached O (10−6) W/kg, which was one order of magnitude higher than the previously observed ε in the upper ocean of the South China Sea slope. At the same observation site, the internal tides induced ε in the bottom boundary layer was O (10−5) W/kg, which indicated that the internal tides played a more important role than the ISWs in enhancing bottom turbulent mixing. • Direct observation reveals strong internal solitary waves near seabed of slope. • Solitary waves cause strong velocity (0.2 m/s) near seabed at depth of 659 m. • Internal solitary waves increase bottom layer turbulent mixing several times. [ABSTRACT FROM AUTHOR]